Project Summary/Abstract This R13 application requests funds to support the XXVth North American Testis Workshop focused on ?Lifelong cell-cell interactions in the testis: a driver for male fertility,? to be held April 3-6, 2019 at The Ritz- Carlton Chicago, Chicago, Illinois. The meeting will be held just before the American Society of Andrology annual meeting, which will take place on April 6-9 at the same location. The Testis Workshop program includes four major lectures (the keynote address and three benchmark lectures), and 15 invited talks that are grouped into five sessions as follows: Cell-cell interactions in testis formation (Session I), Germ cell differentiation in response to local and endocrine factors (Session II), Somatic cell function in response to local factors (Session III), Testicular function in response to man-made chemicals (Session IV), and Pathogen effects on testis function (Session V). After the invited talks, sessions will end with short talks selected from abstracts submitted by young investigators (trainees and Assistant Professors in rank for no more than 2 years). Two poster sessions, with all posters displayed throughout the meeting, are also planned. All attendees are encouraged to submit posters. Since 1972, the North American Testis Workshop has been the premiere international forum for basic and clinician-scientists to present and discuss their recent findings on testis regulation and function. The Workshop attracts 150-200 attendees, approximately 30% of whom are trainees. It also attracts scientists from outside the United States as program organizers, attendees and speakers. The requested R13 funds are intended to help defray travel costs for 8 young investigators who are selected for short talks from submitted abstracts, and part of the housing costs for the 19 invited speakers. The main goals for the Workshop are to: 1) disseminate expert information that will provide better understanding of (a) the development of germ cells in the testes of animals and humans and (b) the role of testicular cell types, hormones and exogenous influences (e.g. viruses, man-made chemicals etc.) on germ cell development and fertility; 2) foster communication and scientific exchange in order to create excellent opportunities for collaboration with peers and ultimately career and scientific growth, thus ensuring that the field of testis biology will continue to advance in the future; and 3) foster the development of the field?s future leaders, with emphasis on trainees, women and underrepresented minorities.